1. Field of the Invention
The present invention relates to an electric vehicle whose wheels are powered by a d.c. power supply comprising a combination of a high-energy rechargeable battery and a rechargeable high-power battery (an electric vehicle such as that powered solely by batteries will be hereinafter simply referred to as an "electric vehicle," as needed), and to a power supply system for a hybrid electric vehicle powered by a combination of a d.c. power supply and an internal combustion engine.
2. Description of the Related Art
FIG. 5 is a view showing a power train of a publicly-known series-type hybrid electric vehicle. In the drawing, reference numeral 1 designates an engine; 2 designates a generator (both the engine and the generator may also be collectively referred to as an "engine generator"); 3 designates a rectifier; 4 designates a main battery; 5 designates an inverter; 6 designates a vehicle drive motor; 7 designates a reduction gear; 8 designates a differential gear; 91 designates a right wheel; 92 designates a left wheel; 10 designates an auxiliary battery; 11 designates a DC-DC converter for recharging the auxiliary battery 10; 12 designates an auxiliary power supply for use with accessories; and 13 designates collectively-grouped accessories such as an air-conditioner.
In terms of efficiency of power generation and a cutback in exhaust gas of an engine, an engine generator is usually operated at an optimum engine speed under optimum load. Specifically, the engine generator is operated in a specified drive condition without reference to a vehicle speed, thereby recharging the main battery 4 via the rectifier 3.
An effort has been made to achieve a further improvement in efficiency and a further reduction in pollution by deactivating the generator 2 when the main battery 4 is recharged to or greater than a specified amount.
In this system, the power of the engine 1 is reduced to a power level smaller than the maximum power generally required to accelerate the vehicle, and the power required to accelerate the vehicle is supplied from the main battery 4. When the power of the engine 1 is greater than a vehicle driving force in constant driving, the main battery 4 becomes recharged by means an excess of power of the engine by way of the generator 2 and the rectifier 3.
When the vehicle is under braking, part of kinetic energy of the vehicle is stored in the main battery 4 through regeneration via the motor 6 and the inverter 5. Simultaneously, the rectifier 3 is operated in inverter mode, and the generator 2 is operated in motor mode. As a result, an engine brake is applied to the vehicle as in the case of an engine-driven vehicle. If there is a deficiency of electric braking force, the vehicle is mechanically braked as in the case of an unillustrated engine-driven vehicle.
Further, when the engine 1 is shut down, the generator 2 does not generate any power. In such a state, the vehicle driving force and the power used for accessories are supplied from the main battery 4.
The power train of the electric vehicle is the same as that shown in FIG. 5, except that the vehicle is not provided with the engine 1, the generator 2, and the rectifier 3. All the power required by the vehicle is supplied from the main battery 4. The main battery 4 is recharged by an unillustrated external power supply.
This electric vehicle does not produce any exhaust gas which would be otherwise produced as a result of operation of an engine, and hence corresponds to a zero-emission vehicle.
As mentioned previously, the publicly-known series-type hybrid electric vehicle is powered by the engine generator in constant driving, and a deficiency of the power required to accelerate the vehicle is remedied by the supply from the main battery 4. In a case where running power required when the engine is in an inactive state or power required by accessories is greater than the power generated by the engine generator, or where the required running power is greater than the power generated by the engine generator even in constant driving, a deficiency of power must be remedied by the power supplied from the main battery 4. For these reasons, the main battery 4 must be formed from a battery which produces high power, i.e., a high-power (or high power density) battery.
The presently dominant main battery for use with a publicly-known electric vehicle is a chemical battery which utilizes a chemical reaction, such as a lead battery, a nickel-hydrogen battery, or a lithium-ion battery. As a matter of course, the main battery is in principle the same as the main battery for use with a hybrid electric vehicle and, hence, is of high power type. There is a desire for a battery which produces greater power.
Next, regenerative braking featuring the hybrid electric vehicle or ordinary electric vehicle will be described. As mentioned previously, when the vehicle is under braking, the kinetic energy of the vehicle is stored in the main battery 4 through regeneration by way of the motor 6 and the inverter 5. The principle characteristic of the electric vehicle of this type is that part of the energy accumulated in the vehicle at the time of an accelerating operation can be regenerated when the vehicle is braked.
The same amount of power is usually generated by braking in a shorter period of time than by acceleration. This means that braking power is greater than accelerating power. Further, the chemical battery usually suffers greater losses at the time of recharge than at the time of discharge.
For these reasons, it is impossible for the hybrid electric vehicle or ordinary electric vehicle to regenerate all the power produced by braking. As things stand, the majority portion of energy produced by braking is converted into heat through mechanical braking.
This means that the bulk of generated power is discarded in the form of heat. Improving the efficiency of utilization of energy by regenerating as much power as possible at the time of a braking operation presents a significant problem.